The first instance of sacrifice in the Bible is a strange one, as we see in Genesis 4:1-6.
“And Adam knew Eve his wife;
and she conceived, and bare Cain,
and said, I have gotten a man from the LORD.
And she again bare his brother Abel.
And Abel was a keeper of sheep,
but Cain was a tiller of the ground.
And in process of time it came to pass,
that Cain brought of the fruit of the ground an offering unto the LORD.
And Abel, he also brought of the firstlings of his flock and of the fat thereof.
And the LORD had respect unto Abel and to his offering:
But unto Cain and to his offering he had not respect.
And Cain was very wroth, and his countenance fell.”
Why did God accept the offering of Abel but not of Cain? Evidently some think that it was because there was no blood offered with Cain’s sacrifice. But this clearly cannot be the case, since offerings to God were often of fruit, bread, and incense in the OT. So what was the reason?
The text does not tell us. We need to be open to the ambiguity that we are presented with. We also need to keep in mind that Cain and Abel were just introduced. They were born, they had respective roles, and then they sacrificed, all in a few verses. Nothing is said of their characters.
One explanation is that Cain harbored ill will against Abel. He goes on to kill him rather quickly in the story, but this could have been a result of jealousy stemming from the sacrifice, not a long held antipathy.
Another explanation is that the story shows the preference for the shepherd over the farmer, the pastoral life over the city life. It should also be noted in this regard that eating animal flesh had yet to be blessed by God (this only happened after the flood). Why was Abel shepherding sheep if nobody was going to eat them? This kind of shepherding is non-violent and non-abusive, while in the world of the readers the shepherd was known to occasionally slaughter a sheep for good (sacrifice) and bad (greed) reasons. The leaders of Israel were often compared to shepherds in the Bible, and criticized for abusing their power and devouring the sheep they were supposed to protect. Shepherds generally did not kill their sheep, but awaited orders from their masters as to when a sheep should be killed. The flock was “on loan” to the shepherd, not owned by him.
This explanation does little to resolve the tension in the story. Various other explanations have been offered, but rather than list all of them I will simply refer to the “The Conflict of Adam and Eve with Satan,” a Christian text that preserves various Jewish and Christian traditions.
The text tells us that Abel loved to pray, fast, and sacrifice, while Cain often would skip out on the action. Cain had a “hard heart,” while Abel had a “meek heart.” Abel was interested in spiritual things, but Cain was interested in ruling over his brother.
The account is very much like Rabbinic retellings of Jacob and Esau. From the womb God favored Jacob, the second born twin, while Esau was “hated.” The two boys grew up very much like Cain and Abel, with one of the pair interested in the fields and hunting (Cain and Esau) and the other stayed at home and was gentle and pious (Abel and Jacob). The sworn enemy of Jacob becomes precisely his twin brother Esau. This continues until the end of time, when Amalek (the anti-Israel figure in the OT who is the descendant of Esau) is destroyed by the Messiah and the face of God is finally revealed to humanity.
In the “Conflict,” Satan appears to Abel while he is praying and tells him that he will kill Abel because he fasts, prays, and offers sacrifices to God. Then Satan appears to Cain, telling him that since Cain’s parents love Abel more than him, they will give Abel Cain’s twin sister for a wife (yes, Cain and Abel were both born with twin sisters in this retelling). Cain wants to marry his own twin (!) rather than Abel’s twin (Abel’s twin was evidently less attractive!). Satan suggests that Cain should kill Abel, leaving his beautiful twin to be his wife.
The story then takes a dramatic turn, in that it begins to conflict directly with the Biblical account. Here Adam tells Cain and Abel both to make an offering to God for their sins: both of their offerings are said to be “the fruit of thy sowing.” They both offer a vegetarian sacrifice. The tension between an animal and vegetable sacrifice disappears.
Abel makes his offering first, but only after being instructed how to do it properly by his parents. God accepts it “because of his good heart and pure body. There was no trace of guile in him.”
Cain, on the other hand, angered his father by not wanting to sacrifice and taking no pleasure in the offering. He offered a sheep (a surprise since the offering was “of his sowing”) but was thinking about eating it as he sacrificed it. He had also picked out the smallest (least desirable) sheep.
We should note here that the text in question has another strange tradition about sacrifice and blood. It tells us that Melchizedek offered bread and wine, and was the first priest. It is specified a number of times that he never offered blood sacrifice, but rather the type of the Eucharist. His priestly ministry was without animal blood, in distinction to the Aaronic priesthood. So the views on animal sacrifice should be seen as later Christian teachings, although they might also be derived from Jewish traditions.
Here we come to the reason for the rejection of Cain’s sacrifice by God: the text tells us that “God did not accept his offering, because his heart was full of murderous thoughts.”
We cannot definitively say why Cain’s offering was rejected, and this is (evidently) how the writer of Genesis wanted it. Had he wanted the meaning to be plain, he would have simply stated it. Instead, ambiguity was inserted into the story, inviting various readers to add their own interpretation to the textual story.
The most convincing explanation is that God did not accept Cain’s sacrifice because he did not accept Cain. Cain was a (future) murderer and was a current murderer in his heart. We have no other information to explain the rejection. The explanatory sections of the “Conflict” fill out this picture with background information: Cain eschews prayer and sacrifice, and is jealous of his brother (the younger favorite).
God rejects his sacrifice because a pure heart is acceptable to God, not sacrifice, as we see from Psalm 50 (51 MT):
“O Lord, open thou my lips; and my mouth shall shew forth thy praise.
For thou desirest not sacrifice; else would I give it: thou delightest not in burnt offering.
The sacrifices of God are a broken spirit:
a broken and a contrite heart, O God, thou wilt not despise.
Do good in thy good pleasure unto Zion: build thou the walls of Jerusalem.
Then shalt thou be pleased with the sacrifices of righteousness,
with burnt offering and whole burnt offering:
then shall they offer bullocks upon thine altar.”
The praise of the lips (prayer) is an acceptable sacrifice, as we also see in Hosea 14:2
“Take with you words, and turn to the LORD: say unto him,
Take away all iniquity, and receive us graciously:
so will we render the calves of our lips.”
Obviously the image is a metaphor, but it shows that the substitution of praise for animal sacrifice was not a development of post-70 CE Judaism, but one that was repeated in the Psalms and Prophets. It is also noteworthy that he Greek version of the verse above has “fruit of our lips,” indicating that the LXX translators saw the “calves” as being “fruit.” The hard distinction between fruit and animals sacrifices cannot be maintained, according to Hosea.
Cain did not have “a broken and contrite heart” while Abel did. Abel offered “sacrifices of righteousness,” which are sacrifices from one who practices righteous acts and has a righteous heart (not merely animal or vegetable offerings). The sacrifices are only acceptable of the sacrificer is acceptable. Cain was not, and it was because his heart was not right with god or his family (all of humanity at that point!).
Therefore the “Conflict” explains the story rather well, even if it of a comparatively late date (5th or 6th century CE). Cain’s offering was rejected because it was not offered with joy, nor remorse, nor charity. Instead, it was offered with a greedy mind, and one bent one killing his brother and anyone else who would stand in his way.